Chromate treated steel plates are widely used as steel plates modified for rust prevention of zinc type plated steel plates. In general, chromate treatments are roughly divided into three types of electrolytic, reaction and coating types.
The electrolytic type produces films of mainly Cr.sup.3+. This film has a high degree of accomplishment, and is sparingly water soluble, and further has an excellent anchoring effect as an undercoat, but this is inferior in corrosion resistance, because Cr.sup.6+ is lacked.
Since the reaction type reduces and precipitates a chromate film on a plated surface by the chemical reaction with Cr.sup.6+ ion and dissolution of a basic metal due to acid, only films of mainly Cr.sup.6+ are obtained. The amount of adhering Cr can be easily increased, but a corrosion resistance could not be improved so much.
In the coating type, a treating solution where inorganic type additives such as silica are added to a basic bath of mainly chromic anhydride, is coated on the steel surface and dried. Since this film contains much Cr.sup.6+, it has the most excellent corrosion resistance and exhibits a high corrosion resistance in response to the adhering amount of Cr. But as Cr.sup.6+ is water soluble, Cr eludes during use of a water soluble paint or a degreasing process as a pre-treating process for painting. Thus, an available adhesion amount of Cr is limited.
As a method of improving the Cr elusion of the coating type, there have been proposed methods where the chromate treating solution is coated, dried, and water-washed away (including a hot water-washing), thereby to in advance elude the water soluble Cr.sup.6+ (Japanese Patent Application Laid Open Specifications No. 202083/87 and No. 202084/87).
Further, known improvements of the Cr elusion dry the chromate solution of the normal coating type at high temperature as shown in the Japanese Patent Publication No. 38891/70 or by lengthening a drying time (CAMP-ISIJ Vol. (1988) 680).
However, since the water-washing of the dried chromate treating solution loses Cr.sup.6+ having a self repairing function, the corrosion resistance is remarkably deteriorated.
For the water-washing method, it will be considered to lower the ratio of Cr.sup.6+ in the chromate solution.
For lowering the ratio of Cr.sup.6+ in the chromate solution, organic reducing agents such as saccharoid or alcohol or inorganic reducing agents are generally employed. If the weight ratio of Cr.sup.6+ /Cr.sup.3+ is not more than 50/50, the chromate solution is gelatinized in a short period of time. The lower limit of Cr.sup.6+ /Cr.sup.3+ ratio for stable use of the chromate solution, has conventionally been around 60/40. But in spite of using the chromate solution reduced to such an extent, the accomplishing degree of a chromate film could not be heightened, and the improvement of the Cr elusion could not be expected.
When the film is dried at the high temperature, the Cr elusion is more or less improved, but the water soluble Cr.sup.6+ is still lowered, and at the same time the chromate film is cracked and the corrosion resistance is decreased. Further for improving the Cr elusion by drying at the high temperature, a temperature exceeding 200.degree. C. is required and problems occur in view of product ion costs.
The present invention has been realized in view of the foregoing problems, and is to provide a method of producing highly corrosion-resistant surface-treated steel plates, where the Cr elusion is little in spite of drying at the low temperature, the chromate film is satisfied with the corrosion resistance, and the surface treated steel plate itself has an excellent corrosion resistance.